SR Latch (Flip-Flop) Circuit
A fundamental digital memory circuit built from two cross-coupled NOR gates.
Circuit Description
The Set-Reset (SR) Latch is one of the simplest sequential logic circuits, capable of storing one bit of information. This implementation uses two NOR gates. The outputs (Q and Q') are "latched" into a state based on the last input signal. It forms the basis for more complex memory elements like flip-flops and registers used in computers.
How It Works
The circuit has two inputs: Set (S) and Reset (R), and two outputs: Q and Q' (Q-not). - Pressing the Set button makes Q HIGH (1) and Q' LOW (0). The latch is now "set". - Pressing the Reset button makes Q LOW (0) and Q' HIGH (1). The latch is now "reset". - If both inputs are LOW, the latch holds its previous state. This is the memory feature. - An invalid state occurs when both S and R are HIGH simultaneously, as it forces both Q and Q' to be LOW, which violates the Q' = NOT Q rule.
Circuit Schematic
Components List
- 1 × 74HC02 Quad 2-Input NOR Gate IC
- 2 × LEDs (different colors recommended)
- 2 × 330Ω resistors
- 2 × Push buttons (momentary)
- 1 × Breadboard
- Jumper wires
- 5V power supply
Installation Instructions
- Place the 74HC02 IC on the breadboard. Connect Pin 14 to +5V and Pin 7 to Ground.
- Connect a push button for 'Set' (S). Wire one side to +5V and the other to input Pin 2 of the IC.
- Connect another push button for 'Reset' (R). Wire one side to +5V and the other to input Pin 5.
- Connect the output of the first NOR gate (Pin 1) to an input of the second NOR gate (Pin 6).
- Connect the output of the second NOR gate (Pin 4) back to an input of the first NOR gate (Pin 3).
- Connect a 330Ω resistor and an LED from Pin 1 (Q output) to Ground.
- Connect another 330Ω resistor and an LED from Pin 4 (Q' output) to Ground.
- Power up the circuit. Pressing the 'Set' button should light up the Q LED, and pressing 'Reset' should light up the Q' LED.